Polyimides, which have excellent properties including heat resistance, dimensional stability, dynamic characteristics, electrical properties, environmental resistance and flame retardance, as well as high flexibility, are widely used as flexible printed boards and tape automated bonding boards employed for mounting of semiconductor integrated circuits. In these fields, polyimide films are used as insulating supports for laminated bodies produced by lamination with metal foils, such as copper foils, via adhesives.
In recent years there has been increasing demand for thinner polyimide films with higher functionality, in the fields of electrical and electronic devices and semiconductors.
Polyimide films with higher rigidity and lower linear expansion coefficients are preferred for better handling properties during working and for higher dimensional precision of laminated bodies obtained by lamination with metal layers, but on the other hand such films also have inferior adhesive properties and give laminated bodies with lower peel strength, thus impairing the reliability of the obtained products. This tendency becomes more pronounced with smaller polyimide film thicknesses.
Many strategies have been proposed in the past to improve the adhesion of polyimide films. For example, it is known that using a polyimide film having a discharge-treated surface improves the peel strength of laminated bodies obtained by lamination with copper foils via adhesives or direct lamination with metal layers (Japanese Unexamined Patent Publication No. 63-61030, Japanese Unexamined Patent Publication No. 2-53932, Japanese Unexamined Patent Publication No. 2-134241, Japanese Unexamined Patent Publication No. 3-56541, Japanese Unexamined Patent Publication No. 5-1160, Japanese Unexamined Patent Publication No. 5-78505, Japanese Unexamined Patent Publication No. 5-152378, Japanese Unexamined Patent Publication No. 5-222219, Japanese Unexamined Patent Publication No. 5-279497, Japanese Unexamined Patent Publication No. 10-12779).
It is also known that using a polyimide film having its surface treated with a surface-treatment agent improves the peel strength of laminated bodies which are obtained by lamination with copper foils via adhesives or direct lamination with metal layers (Japanese Unexamined Patent Publication No. 63-68641, Japanese Unexamined Patent Publication No. 63-99282, Japanese Examined Patent Publication No. 6-2828, Japanese Unexamined Patent Publication No. 9-48864, Japanese Unexamined Patent Publication No. 11-29852).
Moreover, it is known that using a multilayer polyimide film obtained by laminating a polyimide film with another thermoplastic polymer, such as a thermoplastic polyimide thin layer, also improves the peel strength of laminated bodies obtained by lamination with copper foils via adhesives or direct lamination with metal layers (Japanese Unexamined Patent Publication No. 63-218349, Japanese Unexamined Patent Publication No. 6-210794, Japanese Unexamined Patent Publication No. 2003-251773).
However, the peel strengths of laminated bodies produced using such surface-modified polyimide films have been insufficient, and this situation has resulted in reduced productivity while also constituting an impediment to increased device sizes and complicating efforts at manufacture on a commercial scale.
For this reason, there have been proposed in recent years a method of performing treatment with a surface treatment agent after lamination of a polyimide film with a thermo-plastic polyimide thin layer (Japanese Unexamined Patent Publication No. 2000-239423), and a method of performing treatment with a surface treatment agent after treatment of the polyimide film surface by plasma discharge (Japanese Unexamined Patent Publication No. 11-29852).
Nevertheless, there is much need of improvement because these polyimide films do not always provide sufficient peel strength to laminated bodies and, especially, to laminated bodies prepared by lamination with metal foils such as copper foils via adhesives, while organic materials other than polyimides are problematic due to inadequate mechanical properties (tensile modulus) or thermal properties (linear expansion coefficient).